Sealed high-flux neutron tubes are used for the examination of materials by means of fast neutrons, thermal neutrons, epithermal neutrons or cold neutrons.
The tubes which are available at present have an inadequate service life at the level of the emission necessary for realizing their full efficiency in the various nuclear techniques: such as neutronography, analysis by activation, analysis by .gamma. spectrometry of inelastic diffusions or radiactive captures, diffusion of neutrons, etc.
Customarily, the reaction T(d,n).sup.4 He, delivering 14 MeV neutrons, is used most often because of its highly effective cross-section for comparatively low neutron energies, but any other appropriate reaction may also be used.
However, regardless of the type of reaction, the number of neutrons obtained per unit of charge in the beam always increases as the energy of the ions directed toward a thick target itself increases. This phenomenon is more pronounced beyond ion energies obtained in the sealed tubes available at present which are powered by a high voltage (THT) which rarely exceeds 200 kV for reasons of tube definition as well as for reasons of reliability of high voltage generators and connection members.
Some of the most important phenomena restricting the service life of a neutron tube are the irradiation faults of the target by the incident ions and the metalization of the insulating walls of the tube.
Because these two phenomena are more significant as the intensity of the beam itself is higher, it would be important to limit this parameter to a maximum value and hence to use high acceleration voltages for a given neutron emission.
Unfortunately, contrary to vacuum tube's (for example, X-ray tubes) in a conventional sealed neutron tube it is not possible to increase the dimensions of the tube in practice. This this would on the one hand lower the neutron yield and on the other hand cause ignition of discharges in accordance with Paschen's law in the low pressure range.
Another risk of igniting discharges in the gas is due to the surface effect of electrodes exposed to a high electric field. This effect is initiated by electric particles emitted by a part of the tube carrying a negative potential and acting as a cathode which faces another part of the tube which carries a positive potential and thus acts as an anode. This is not to be confused with the parts of the tube bearing identical denominations such as, for example, the anode and the cathode of the ion source. These particles strike other molecules of the material in the gas, or on the electrodes, and may cause, by secondary emission, a given amplification of the emission, and thus progressively form an electric current which is sufficiently large to cause a breakdown by disturbing the dielectric qualities of the surroundings, either on the surface of the insulating parts of the tube or across the gaseous space of the tube itself. When the described reaction T(d,n).sup.4 He is used, the presence of the tritium emitter .beta..sup.- further increases this risk, just like the various ionising radiations associated with the nuclear reaction (X, .alpha., .gamma., n) or with its consequences (radiation induced by neutron activation of the tube itself or of its environment).
In vacuum tubes such as, for example X-ray tubes, notably the breakdown behaviour on the surface of insulators is improved on the one hand by increasing the electrode spacing and sub-dividing the tube into two parts which constitute the anode and the cathode, respectively, so as to reduce the mean potential in each part of the tube, and on the other hand by imparting an inclination to the insulating parts which is adapted to the direction of the electric field (see, for example the article entitled "Metal/ceramic X-ray tubes for non-destructive testing" by W. Harth et al, published in Philips Technical Review, Vol. 41, 1983/1984, No. 1, pp. 24-29).
Neutron tubes are gas-filled tubes whose contents are under a low pressure so that the product P*d of the pressure and the electrode spacing is situated at the left of the Paschen curve. In that case discharging phenomena can occur, notably of the Townsend avalanche type, which can be avoided by reducing the electrode spacing; this approach, however, is limited by the threshold for the appearance of a strong cold emission of electronic origin according to the Fowler-Nordheim law (F-N).
For a given potential difference cold emission current density values calculated by way of the Fowler-Nordheim formula produce, in dependence of the surface states of the electrodes, a high amplification coefficient for this current density for a given potential difference. As a result, a slight voltage variation can produce a strong increase or decrease of the current, depending on the direction of this variation. Qualitatively, such a strong sensitivity of the current to the voltage is observed for all parasitic phenomena causing a current between the electrodes.
Thus, beyond a given voltage theshold it becomes difficult to avoid ignition in the gas either by the surface effect of the electrodes subjected to a high electric field, or by collision of ions with the gas molecules when the insulating distance is increased in order to reduce the electric field.